Drag or wind resistance is a major force acting against the movement of a cyclist. Greater drag requires a cyclist to exert more energy to maintain or increase speed. This is particularly important in bicycle racing in which riders must conserve energy over long distances and races can be won or lost in a matter of seconds.
One way in which to reduce drag is to make components more aerodynamic. In the past 20 years, new designs have been developed to reduce the drag caused by airflow over the wheels. Some manufacturers have produced rims so that the rim and tire form an oval (elliptical) shape with the rim being narrower than the tire at the outer edge of the rim and widest part of the rim occurring at the minor axis of the ellipse. This shape has been modified in some products to have parallel braking surfaces near the outer portion of the rim.
Such wheels are made with the assumption that the airflow will be head-on to the cyclist. In practice, however, side winds cause air flow to come from angles to the side of the wheel, causing previous wheels to stall and lose aerodynamic efficiency.